1. Field of the Invention
The present invention relates to a device on a sewing machine for applying a tensile stress to a material to be used in the sewing process as it is being fed, such as a thread or a ribbon. It relates more particularly to a sewing machine having guides for the material, means for generating a controlled frictional force acting on the material, a forcemeasuring device, and a control unit connected to the force-measuring device and the force generating means.
2. Background Art
One such device is disclosed in Federal Republic of Germany Patent 28 09 848 (corresponding to U.S. Pat. No. 4 289 087) in which a thread-tensioning device has a setting drive for applying a force to pressure disks, a pressure sensor for detecting the force exerted by the setting drive, and a control responsive to the pressure sensor for regulating the tensile force applied to the thread. One problem arises from the fact that the tensile force equals the product of the force times the coefficient of friction. Since the control's function is exclusively to maintain the force constant, this device has the disadvantage that the coefficient of friction remains unconsidered, so that while the thickness of the thread to be tensioned is considered by the control, its sliding frictional properties are not. In addition, in the known device, variations due to varying speeds of passage are not taken into account. This leads to the necessity of readjusting the device when the sewing machine is loaded with a thread having different sliding frictional properties. Furthermore, because of these drawbacks, this device is only conditionally suitable for use in a programmable sewing system in which the tension of the thread is intended to be varied, as one of the operating parameters in a sewing program.
Federal Republic of Germany Patent 26 06 35 discloses a device for measuring thread tension in a sewing operation in which a thread tension feeler is arranged in the region between a thread tensioner and the stitch-forming area of the sewing machine. The known device serves to detect tension irregularities when stitches are being formed and is used to generate a stitch-error signal which is provided to a corresponding indicating device. That patent does not disclose any means for controlling the tensile stress to be applied to a thread in order to control the thread tension.
An arrangement for measuring thread, ribbon or wire tension is disclosed in Federal Republic of Germany Patent AS 12 73 861, in which a piezoelectric crystal is used for the measurement process. This patent generally suggests using the disclosed arrangement as a measurement-value transmitter as part of a control system which includes a brake which acts on the thread.
According to another document, U.S. Pat. No. 2,810,532, it is also possible to apply a tensile force to a material to be sewn by using a so-called drum-type tensioning device.
The disclosures of the prior art materials mentioned herein are expressly incorporated by reference.